An independent, government-appointed watchdog in the United States has opened an investigation into the Secret Service’s handling of security for Donald Trump on the day the former US president was shot during a Pennsylvania rally.

In a brief notice on the US Department of Homeland Security’s inspector general office’s website, the agency said the probe aims to “evaluate the [Secret Service’s] process for securing” Trump’s July 13 campaign event.

The notice, which was on a list of “ongoing projects” that the inspector general’s office is looking into, did not say exactly when the investigation was launched.

The move comes as Saturday’s attempted assassination against Trump, the Republican Party’s 2024 presidential nominee, continues to roil the country as it prepares for presidential elections in November.

Trump was shot in the ear by a gunman that witnesses say had taken up a position on a rooftop with a direct line of sight of the rally stage, raising questions about what security measures were put in place before the event.

US President Joe Biden, who is set to face off against Trump in the upcoming election, has ordered an independent review of security protocols at the rally, where one attendee was killed and two others were seriously injured.

But US lawmakers are pushing for more immediate answers from the heads of key government agencies, including Homeland Security Secretary Alejandro Mayorkas, FBI Director Christopher Wray and Secret Service Director Kimberly Cheatle.

Earlier this week, Republicans on the US House of Representatives’s Homeland Security Committee sent letters to the three leaders, asking them to testify before the panel on July 23.

“The American people deserve answers,” the Republicans wrote in a social media post.

Separately, US senators are expected to be briefed by the Secret Service, FBI and Department of Justice on Wednesday afternoon, local media outlets have reported.

Cheatle at the Secret Service also will be questioned at a House Committee on Oversight and Accountability hearing on July 22.

“The United States Secret Service has a no-fail mission, yet it failed on Saturday when a madman attempted to assassinate President Trump, killed an innocent victim, and harmed others,” the panel’s chairman, Congressman James Comer, said in a statement.

BREAKING: Chairman @RepMarkGreen has requested Secretary Mayorkas, Director Wray, & Director Cheatle testify before the Committee on July 23 re: the attempted assassination of President Trump.

Our info requests must be satisfied by July 19.

The American people deserve answers. pic.twitter.com/U9nzsCiw9W

— House Homeland GOP (@HomelandGOP) July 16, 2024

“We are grateful to the brave Secret Service agents who acted quickly to protect President Trump after shots were fired and the American patriots who sought to help victims, but questions remain about how a rooftop within proximity to President Trump was left unsecure,” he said.

“Americans demand answers from Director Kimberly Cheatle about these security lapses and how we can prevent this from happening again.”

In an interview with ABC News on Monday, Cheatle addressed reports that the suspected gunman, 20-year-old Thomas Matthew Crooks, had been seen by witnesses shortly before the shooting began.

“I don’t have all the details yet, but it was a very short period of time,” she told the US news outlet. “Seeking that person out, finding them, identifying them, and eventually neutralising them took place in a very short period of time, and it makes it very difficult.”

The Secret Service chief also said local authorities were tasked with securing the building from which the shots were fired.

“In this particular instance, we did share support for that particular site,” Cheatle said, explaining that “the Secret Service was responsible for the inner perimeter”.

“And then we sought assistance from our local counterparts for the outer perimeter,” she said. “There was local police in that building – there was local police in the area that were responsible for the outer perimeter of the building.”

Local authorities have pushed back, however, amid questions about which law enforcement agency was ultimately responsible for securing the rally in Butler, Pennsylvania.

Richard Goldinger, the Butler County district attorney, told The Washington Post this week that the “Secret Service was in charge, and so it was their responsibility to make sure that the venue and the surrounding area was secure.”

“That’s common sense, I think. That’s their job,” Goldinger said.